1. Field of the Invention
The present invention relates to a network system, a communication device, and a communication routing method for transferring data from a sever to a client over either one of a two-way communication line for transmitting data between the server and the client bidirectionally and a one-way communication line for transmitting data in only one direction from the server to the client.
2. Description of Related Art
Internet connections via a communication satellite have recently been developed for commercial use as a medium for realizing rapid access. FIG. 10 shows an outline of an Internet connection over a communication satellite. A client 10 is connected to a telephone line and then to the Internet via a provider 22 in the same manner as in a common Internet connection. Two-way communication between the provider 22 and the client 10 is possible, and in many cases, its data transfer rate is approximately 64 kbit/s. The connection between the client 10 and the telephone line is, as shown in FIG. 11, established via a telecommunications gear, such as a modem 32 and a terminal adapter.
In Internet connections using satellite communication, it is possible to receive data over a satellite line, which is usually received over a telephone line. The satellite line is one-way wireless communication from a transmitter 26 for satellite line to the client 10 via a communication satellite 24, and its data transfer rate is usually around 1 Mbit/s. As shown in FIG. 11, a parabolic antenna 38 for receiving data transmitted from the communication satellite 24 by wireless communication and a satellite communication receiver 36 for demodulating data received by the parabolic antenna 38 are connected to the client 10.
In general Internet usage, data volume transmitted from a server 20 to the client 10 is much greater than the data volume transmitted from the client 10 to the server 20. Accordingly, the use of a satellite line is very effective.
It is, however, difficult for a satellite line to be upgraded and newly installed because of the use of the communication satellite 24. Since there is a connection between the provider 22 and the client 10 at the ratio of 1:1 via a telephone line, the data transfer rate is roughly fixed. Since a satellite line between the communication satellite 24 and the client 10 is allocated to each user at the ratio of 1:n, as the number of users is getting larger, the slower its data transfer rate becomes. In addition, the transfer rate of the satellite line easily changes depending on the weather.
Further, longer response time is required for the requested data to be received by the client 10 over the satellite line after sending a data transfer request to the server 20. As shown in FIG. 12(a), total data transfer volume of the telephone line becomes greater shortly after starting data transfer due to short response time. The transfer rate of the satellite line is generally faster than that of the telephone line, so that the final transfer time over a satellite line is faster. On the contrary, when there is a small volume of data to be transferred or the data transfer rate of the satellite line is slow due to a large number of satellite line users, as shown in FIG. 12(b), the final data transfer time of the telephone line may be faster than that of the satellite line.
It is possible to select a communication route for data transfer in Internet connections employing a conventional two-way communication line. For example, as shown in FIG. 13(a), it is assumed that the client 10 and the server 20 are connected by a communication route α via router A, router B, and router C and a communication route β via router A, router D, router E, and router F. When transferring data from the server 20 to the client 10, the server 20 specifies the forwarding address of the data, but in general, it does not particularly specify a data transfer route.
Router A selects a data transfer route in FIG. 13(a). Router A basically selects a communication route so that the number of routers that pass through to the data forwarding address may be minimum. The communication route α via router B and router C is selected in FIG. 13(a). In addition, since it is possible to conduct communication among routers on information such as data transfer rate and the like, router A can select a communication route according to the data transfer rate and the like. For example, if the data transfer rate between router C and the client 10 substantially slows down, the communication route β via router D, router E, and router F may be selected.
However, as shown in FIG. 13(b), since a satellite communication line is a one-way communication line from the communication satellite 24 to the client 10, it is impossible to carry out communication on data transfer rate and the like. In the case of a satellite communication line, the client 10 usually specifies routers used in data transmission and then the server 20 transfers data via these specified routers. In FIG. 13(b), either one of router C (communication route α) and router F (communication route β) is specified. The client 10, however, usually specifies router B (communication route α) to request for data transfer because a satellite line is generally faster than a telephone line. When a satellite line is used, it is impossible for router A to select a communication route.
It is an object of the present invention to provide a technique for dynamically selecting an optimum communication route based on estimated transmission speed over different communication lines.